1. Field of the Invention
The present invention generally relates to audio systems and, more particularly, to a ported enclosure and automated equalization of frequency response in a micro-speaker audio system.
2. Description of the Related Art
Conventional loudspeakers are typically mounted within an enclosure where the enclosure blocks the rear-generated speaker sounds which are out of phase with the sounds being directed to the listener by the speaker. Mounting the loudspeakers within an enclosure prevents reduction of speaker volume output, as detected by the listener, caused by cancellation between the sounds emanating from the front and back sides of the loudspeaker cone.
The enclosure to which the loudspeaker is mounted acts as a baffle, where the dimensions of the baffle determine the range of frequencies at which the baffle is effective. A baffle appears to be of infinite size if the baffle limits the radiation pattern of the loudspeaker from 360 degrees to 180 degrees. Under these circumstances, someone standing behind the baffle hears essentially no sound, while someone standing in front of the baffle hears essentially twice the sound volume as compared to the sound output without this “infinite baffle” effect. Such an infinite baffle results in a 6 db, or 2×, boost in sound pressure as compared with a non-baffled loudspeaker. Generally, the baffle appears to be infinite, and thus effective, at frequencies whose wavelengths are no greater than approximately three times the shortest dimension of the baffle. For speakers mounted in mobile, or handheld, devices, the baffle size is typically determined by ergonomics and industrial design considerations.
The baffle appears to be finite, and thus ineffective, at frequencies whose wavelengths are greater than approximately three times the shortest dimension of the baffle. A finite baffle size poses a problem for handheld devices. If a handheld device is placed on a table, then the table acts as an infinite baffle even at fairly low frequencies. For example, if the shortest dimension of the table is 24 inches, then the table would act as an infinite baffle at all wavelengths up to 3×24 or 72 inches, corresponding to frequencies at or above approximately 187 Hz. However, if the shortest dimension of the handheld device is four inches and the device is being held in free-space, then the device would act as an infinite baffle only up to wavelengths of 3×4 or 12 inches, corresponding to frequencies at or above approximately 1125 Hz. In this example, the mobile device held in free air would have an output that is reduced by 6 db at frequencies below 1125 Hz; whereas, if the same device is placed on a table, then this 6 db reduction would occur only for frequencies below 187 Hz. In other words, if such a handheld device is being held in free-space, then the loudspeaker volume and performance of the handheld device would be reduced in the range of 187-1125 Hz as compared with the performance of the device when placed on a table. Effectively, sounds below 1125 Hz would appear to be dampened or suppressed as compared to sounds above 1125 Hz.
In addition, conventional loudspeakers are often placed within a ported enclosure, also known as a vented enclosure or bass reflex enclosure, which may reduce certain distortion effects as compared with such a speaker placed within a sealed enclosure. Such a ported enclosure may reduce the effective displacement of the loudspeaker, particularly when the loudspeaker is driven at loud volumes in the lower portion of the frequency range. Distortion occurs when the actual displacement of the loudspeaker exceeds the speakers rated maximum displacement limit. By reducing the effective displacement of the loudspeaker, the loudspeaker may be driven to higher volumes before the speaker starts producing distorted sound waves. Accordingly, a loudspeaker in a ported enclosure may be driven to louder volumes without low frequency distortion as compared to the same loudspeaker in a sealed enclosure, thus improving the low frequency output of the speaker enclosure.
As the foregoing illustrates, what is needed in the art is an improved approach for producing sound in a handheld device.